Many electronic devices include or are connected to amplification devices that amplify input signals and generate amplified output signals. As is well known in the art, these amplification devices may increase or decrease the signal level of the input signal in accordance with a gain. FIG. 1 illustrates a generalized example of a prior art amplification device 10. The amplification device 10 includes an amplification circuit 12 and biasing circuitry 14. The amplification circuit 12 amplifies an input signal 16 and generates an amplified output signal 18. The biasing circuitry 14 generates a biasing signal 20 that may be applied to the input signal 16 to set a bias level and place the input signal 16 within an operating range of the amplification circuit 12. FIG. 2 illustrates the input signal 16 after application of the biasing signal 20. In this example, the input signal 16 and biasing signal 20 are voltage signals and the biasing signal 20 is a DC voltage that provides a DC bias voltage level 22.
In some applications, the signal levels of the input signal 16 are increased and decreased in bursts. As illustrated in FIG. 2, the voltage level of the input signal 16 varies from Viped_h to Viped_l relative to the DC bias voltage level 22 between times t0 to t1 but is increased rapidly to vary between Vimax_h to Vimax_l between times t1 to t2. The input signal 16 may then suddenly decrease once again to vary from Viped_h to Viped_l after time t2.
FIG. 3 illustrates a voltage of the amplified output signal 18 that is output from the amplification circuit 12 as a result of the input signal 18 in FIG. 2. The quiescent operating level of the amplified output signal 18 is the level of the amplified output signal 18 when no input signal 16 is received by the amplification circuit 12. In the amplification device 10, the quiescent operating level is initially set at QI by the DC bias voltage level 22 (shown in FIG. 2) from t0 to t1. Also, the amplified output signal 18 varies from Voped_h to Voped_l from t0 to t1. Ideally, the quiescent operating level of the amplification device 10 remains consistent QI through all of the bursts. However, when a burst in the input signal 16 causes a burst in the amplified output signal 18 at t1, the amplified output signal 18 varies from Vomax_h to Vomax_l and the quiescent operating level of the amplification circuit 12 drifts from QI to QN. This drift in the quiescent operating level prevents the amplified output signal 18 from quickly returning back to varying from between Voped_h to Voped_l at time t2. It is not until time t3 that the quiescent operating level returns to QI.
FIG. 4 illustrates an average power curve 24 of the amplified circuit 12 resulting from the amplification of the input signal 16 in FIG. 2. The power level of power curve 24 is at P_ped between times t0 to t1 but is increased to P_max between times t1 to t2. Section 26 illustrates the ideal behavior of the power curve 24 where the power curve 24 changes almost instantaneously from P_max to P_ped at t2. However, the drift in the quiescent power level from QI to QN (shown in FIG. 3) causes the output signal to instead behave as illustrated in section 28 at time t2. As shown by the section 28, the power curve 24 does not instantaneously change back from P_max to P_ped. Instead, the power curve 24 does not reach P_ped again until time t3.
This drift in the quiescent power level of the amplified output signal from QI to QN may be caused by the heating of components within the amplification circuit 12. FIG. 5 illustrates a more detailed embodiment of the prior art amplification device 10. As shown in FIG. 5, the amplification circuit 12 may include a transistor 30 having a gate terminal that receives the input signal 16 and has been biased to the bias level 22 by the biasing signal 20 from the biasing circuitry 14. In this example, the amplified output signal 18 is the voltage from the collecting terminal to the emitter terminal of the transistor 30. When the input signal 16 bursts to vary from Viped_h to Viped_l to Vimax_h to Vimax_l the transistor 30 begins to heat up. As the transistor 30 heats up, this causes the quiescent operating level to drift from QI to QN because the heating of the transistor 30 causes a decrease in the base-emitter voltage. As a result, the current from the collecting terminal to the emitter terminal of the transistor 30 increases. Thus, the power curve 24 of the amplification circuit 12 (shown in FIG. 3) and the voltage of the amplified output signal 18 (shown in FIG. 4) are held above their respective ideal levels P_ped and QI until the transistor 30 can cool down.
This drift of the quiescent operating level of the amplification circuit 12 may be referred to as a local thermal memory effect. At high output powers the power dissipated by the amplification circuit 12 may cause the temperature of the transistor 30 to rise to be up to be significantly hotter than the temperature of other components in the amplification device 10, such as biasing circuitry 14. Once the output power is reduced, the transistor 30 cools down at a rate dictated by the thermal time-constant of the material it is built on.
FIG. 6 illustrates a temperature 32 of the transistor 30 as the input signal 16 bursts to vary from Viped_h to Viped_l to Vimax_h to Vimax_l (shown in FIG. 2). When the input signal 16 first bursts to vary from Viped_h to Viped_l to Vimax_h to Vimax_l at time t1, the temperature of the transistor 30 increases very quickly. At time t2, when the input signal 16 bursts back to vary between Vimax_h to Vimax_l the temperature of the transistor 30 does not change instantaneously and there is a direct correlation between the temperature 32 of the transistor 30 and the amount of time it takes for the power level 24 (shown in FIG. 4) and voltage (shown in FIG. 3) to return to their respective ideal levels P_ped and QI, respectively.
As response times for electronic devices become increasingly smaller, amplification devices must also provide quicker transitions from one state to another. Often output signals must change in accordance with masking constraints and thus the drift in the quiescent operating level of the amplified output signal can prevent the amplification device from meeting these masking constraints. Thus, what is needed is an amplification device that reduces, compensates, and/or eliminates the thermal memory effect and maintains a more consistent quiescent operating level of the amplified output signal throughout the operation of the amplification device.